This invention relates to a light-sensing, light fixture control system for conserving electrical energy and for providing a more comfortable lighting level. The present invention is useful whenever a varying amount of ambient light, usually from the sun, provides supplemental lighting. This applies to indoor lighting, where windows, skylights, or doors are available, as well as to parking areas or other outdoor lighting situations.
It has been found that electric lights are often misused by their occupants, especially in commercial buildings. People fail to discipline themselves or to remember to turn off the lights when there is adequate or more than enough ambient light. With the high cost of electrical power today and its expected increase in cost in the future, it is increasingly important to utilize natural light from the sun to the greatest extent possible by controlling supplementary electrical lighting.
Various systems have been devised in the prior art to meet these ends. Timer controllers are satisfactory for some outdoor lighting needs, but not for indoor work space lighting as they cannot accommodate differing weather conditions. Further, a timer requires seasonal adjustments to compensate for changes in times of sunrise and sunset. The more sophisticated and complicated energy management systems use a computer clock controller; such systems are expensive to install, particularly in an already existing lighting system. A large number of light fixtures must be operated by a computer system to offset the expense of the computer, other equipment, and installation. Also, such systems do not automatically take into account seasonal or weather changes or changes in the amount of light admitted by window treatments such as venetian blinds and draperies.
The simplest systems typically use a roof-mounted photocell connected to a bi-metal relay, which controls high voltage power to a lighting circuit. Material costs for this type of system are low, which makes it fairly economical even when only a few fixtures need to be controlled. However, the simple systems have several disadvantages. The sensing unit usually has a very limited light sensitivity range of approximately 5 to 30 foot-candles. In the higher light ranges, the simple systems will not perform satisfactorily and usually will not work at all in window light control in the 200 foot-candle range. Adjustments may require trips to the roof with the concomitant time and expense. Because of state and national electrical safety codes, the high-voltage circuitry requires expensive installation methods, i.e. special conduit and a licensed electrician. Additionally, the simple controllers have the capability of sensing only a single level of ambient light intensity. Below this level, the light is turned on; above this level, the light is turned off. Therefore, they cannot be used where the lighting fixture which is being controlled affects the ambient light intensity, because they will cause the light to quickly cycle on and off.
Therefore, it is desirable to have a low-cost system that can be installed in existing buildings without meeting electrical code requirements for high voltage wiring, that can be easily and unobtrusively mounted, and yet provide a reliable, accurate and sensitive control for turning off light fixtures.
It is particularly desirable to include in such a system a high- and low-level, ambient light sensor whereby the lighting level at the actual work place can be monitored and controlled.
It is further desirable that such a system include buffering so that transient changes in light level such as temporary shadows, clouds and reflections will not cause a corresponding transient response in the light fixtures.
It is further desirable that such a system incorporate a "fail-safe" feature that insures that the light fixture circuit will be closed if the system malfunctions or is disconnected.